The invention relates to a rotational angle sensor, and in particular to a rotational angle sensor for detecting the rotational position of a steering shaft of a motor vehicle, which can carry out several rotations.
DE 10 2005 007 259 B4 describes a position measuring device having a rotatable shaft and a magnetic ring coupled with the shaft by way of several springs, and a magnetic yoke arranged on the outer circumference of the magnetic ring. The magnetic ring can have north and south poles alternately distributed in a circumferential direction. Upon rotation of the shaft, the movement of the magnetic ring initially lags behind that of the shaft due to the magnetic forces until the rotational forces exerted by the springs are greater than the magnetic forces, whereupon the magnetic ring is abruptly moved, releasing kinetic energy, which generates electrical impulses by way of a coil arranged on the magnetic yoke, which are counted in an evaluation device and are stored in a non-volatile memory. Thus, the counted electrical impulses are a measure for the angle of rotation of the magnetic ring. Due to the non-volatile memory, the most recently stored value can be retrieved even in the event of failure of the supply voltage. However, if the magnetic ring rotated during the failure of the supply voltage, the stored value of the rotational position no longer corresponds with the actual value, and all subsequent measurement results are incorrect.
From DE 199 62 241 A1 and DE 195 06 938 A1, rotational angle sensors are known, in which two measuring gear wheels are coupled with a gear wheel of the shaft, wherein the two measuring gear wheels have differing numbers of teeth, specifically such that in the predefined measuring range, one of the measuring gear wheels carries out a number n of full rotations, whereas the second measuring gear wheel runs through a different number of n−1 or n+1 of full rotations. Two sensors scan the rotational position of the two measuring gear wheels and generate corresponding output signals, from the phase difference of which an absolute position signal can be determined.
With these rotational angle sensors, the accuracy of the measurement depends, among other things, on the precision of the toothing. Due to manufacturing tolerances, installation tolerances with respect to the axle distances, and wear and tear, gear wheels can have a clearance, which in a more general sense can also be called a slip, and which leads to measurement errors. In the event of a reversal of the direction of rotation, this error manifests itself as hysteresis error. A further error source with gear wheels and gear rods is the lack of uniformity in the form of the teeth and the tooth spaces, and also in non-uniformity in the distribution of the teeth, which leads to linearity errors even if the engagement of the teeth is free of play. This also holds true for non-circular gear wheels, with the result that in certain rotational position ranges, a clearance is present, and in other rotational position ranges there is not. It is thereby to be taken into account that in most measuring applications plastic gear wheels are used.
To solve this problem, DE 199 62 241 A1 proposes to tension the measuring gear wheels in a spring-loaded manner against the gear wheel of the shaft.
DE 199 37 120 C2 describes a steering angle measuring device having a buffer spring that is spirally arranged around the steering column of a vehicle, one end thereof being arranged on the steering column and the other end thereof being stationary, wherein on the buffer spring, a measuring element for measuring the strain and/or bending of the buffer spring is arranged. From the measured strain and/or bending of the buffer spring, the steering angle is determined.
DE 198 35 886 A1 describes a steering angle sensor having a buffer spring, which is likewise fastened to the steering shaft and to the housing. A sensor detects the change in the inductance of the buffer spring, which is caused by a change in the number of windings. The buffer spring is a flexible flat strip of resilient material, which frequently is also referred to as clock spring.
DE 42 33 393 A1 proposes to provide a spring membrane, which is fastened to a housing and to a travelling nut that is slidable alongside the shaft. Strain gauges detect the deformation of the spring membrane.
DE 198 05 106 C1 shows a rotational position sensor having a rotatable shaft and a fixed stator surrounding the same. In the gap between the shaft and the stator, two series connected buffer springs having opposite winding directions are arranged, in the connecting region of which a covered electrode is arranged, which in interaction with a fixed electrode on the stator forms a capacitor. When the shaft rotates, the two electrodes change their relative position to one another and cause a change in capacitance of the capacitor, which is a measure for the rotation of the shaft relative to the stator.
Modern motor vehicle electronics need a highly precise angle of rotation signal for the rotational angle of the steering shaft. That is to say, good linearity, absence of hysteresis, high resolution capacity, and high measuring speed are required. Resistance to wear and tear, and long service life are likewise inherent criteria, as are low unit costs.
In order to meet these criteria, the sensors having measuring gear wheels as described above are very laborious and expensive. The sensors working with springs as described above do not deliver the required measuring accuracy, and in particular, not the required resolution capacity.